Different kinds of treatment of material by which the treated substrate's content of sugars and the like are made more available have been described in literature. The most well-known are: a) Strong and weak acid hydrolysis, b) wet explosion (Steam Explosion—STEX), c) wet oxidation (WO), d) basic fibre explosion (Ammonia Fibre Explosion—AFEX), e) thermal hydrolysis (Liquid Hot Water—LHW) and e) thermal hydrolysis with addition of base and oxidizing agent.
Strong and Weak Acid Hydrolysis
The described types of strong and weak acid hydrolysis are characterized in that hemicellulose is hydrolyzed and thereby dissolved concurrently with the availability of cellulose being increased for a subsequent acid-based or enzymatic hydrolysis. When using these methods it is, after separation of the insoluble and the dissolved fractions, possible to process these fractions further among others by means of fermentation.
The strong acid hydrolysis has among others been described by Lightner (U.S. Pat. No. 6,258,175), where also the possibility of re-using the applied acid after precipitation with ethanol is described. The primary purpose of the process is to dissolve cellulose and hemicellulose for subsequent use in e.g. production of ethanol by means of fermentation. U.S. Pat. Nos. 6,022,419, 5,705,369, 5,503,996, 5,424,417, 5,125,977 and FR 2.580.669 describe the weak acid hydrolysis (single- and multi-step process) that is used in the separation of cellulose and hemicellulose from the other components in the biomass. Hemicellulose is dissolved in the acid hydrolysis, and a smaller fraction of the lignin is furthermore dissolved. The described processes moreover comprise separation of hemicellulose (in dissolved form) and cellulose (as solid fraction).
There are several problems connected with acid hydrolysis of biomass. Firstly it is necessary to divide the material to very fine particles (<1 mm), which is extremely energy demanding. Furthermore, a neutralization of the treated material is required, which is normally carried out by addition of CaCO3 (limestone). This means that the consumption of chemicals in the process is high concurrently with a considerable amount of hydrated calcium sulphate being accumulated by the neutralization process. Moreover, the treated material from the acid hydrolysis has an inhibiting effect on enzyme hydrolysis and microbial fermentation compared to material resulting form other forms of treatment (see below). Finally, pumps, reactors and the like are exposed to corrosion as a result of the acid-catalyzed process.
Wet Explosion
Wet explosion (STEX) was described as far back as 1928, where Mason developed the process for manufacturing hardboards (U.S. Pat. No. 1,824,221 and U.S. Pat. No. 2,759,856). The STEX process consists of thermal hydrolysis under high pressure, whereafter the pressure is released in a so-called “flash effect”, where an explosion of each fibre takes place due to the great drop of pressure—hence the name wet explosion (or steam explosion). This method of treatment has later on been further developed for the manufacture of e.g. ethanol (Morjanoff and Gray 1987) and paper (WO 98/27.269). GB patent application 2.145.090 relates to a three-step hydrolysis process for the treatment of lignocellulosic organic material by means of high temperatures and pressure, by which pentose-, hexose- and lignin-fractions are separated in three separate steps. Moreover, the process comprises cooling by “flash evaporation” with the aim of preventing the resulting sugars from dissociating. However, these process steps require a high degree of mechanical division and do not comprise the addition of oxidizing agents, the use of which have shown to be advantageous in connection with methods according to the present invention.
In STEX normally a partial dissolution of hemicellulose (>80%) takes place, and cellulose is made available for subsequent hydrolysis. The effect of STEX resembles the effect of acid hydrolysis—however, the STEX process exposes the process equipment to far lesser wear and is not so demanding as regards the use of chemicals and accumulation of waste. However, in STEX there is still a considerable formation of substances that inhibit a possible subsequent fermentation process (Palmqvist and Hahn-Hägerdal 2000) particularly when the material previously has been liquified with acid (SO2 or H2SO4 (Martin et al. 2002)). Furthermore, no noticeable decomposition of lignin takes place in STEX, wherefore the lignin is still able to effect a possible enzymatic hydrolysis.
Wet Oxidation
Wet oxidation (WO) has been developed in order to oxidize organic waste fractions (U.S. Pat. No. 2,690,425) and has later on been modified so as to obtain a solution of hemicellulose from lignocellulose-containing biomass and organic waste (WO 0014120). Wet oxidation comprises a thermal process with addition of an excess of pressure of oxygen as well as a basic catalyst, whereby the hemicellulose is partially dissolved and a part of the present lignin oxidized. Hereby the availability of cellulose is increased. Compared with STEX and acid hydrolysis, in WO only a partial dissolution of hemicellulose takes place (Bjerre et al. 1996). Normally, WO does not require an extra process step for the removal of inhibiting substances. Klinke et al. (2002) describes that the concentration of these inhibiting substances is considerably smaller in a wet oxidation process compared with STEX and acid hydrolysis. WO 0014120 describes a method for dissolution of hemicellulose in lignocellulose-containing materials (primarily leguminous plant sources). The method comprises heating of biomass in an aqueous medium in the presence of an oxidizing agent, in this case oxygen. However, this method does not, as the methods according to the present invention, comprise a “flashing process”. The wet oxidation process has previously shown to be effective at biomass concentrations over 100 g of dry matter/I or on material having particle sizes above 10 mm. Both of these limitations are damaging for the process economy when producing af large-scale. Neither do the described processes permit a re-use of steam, which again has a negative influence on the process economy.
Basic Fibre Explosion
Basic fibre explosion (AFEX) is a process that combines steam explosion and addition of a basic catalyst for treatment of different types of biomass for improving feedstuff or for a further fermentation to e.g. ethanol (U.S. Pat. No. 5,171,592). U.S. Pat. No. 5,865,898 describes a process for treatment of lignocellulose-containing biomasses, which comprises addition of calcium oxide or calcium hydroxide and an oxidizing agent followed by heating to a relatively high temperature (however always below 100° C. in order not to decompose the lignocellulose-containing biomass). In traditional AFEX the biomass is liquified in ammonia water at moderate temperatures (˜50° C.), after which the pressure is momentary released (explosion). By this process cellulose and lignin are modified, which makes the cellulose more reactive (available), concurrently with release of the hemicellulose. The process creates considerably fewer inhibiting substances than the acid-catalyzed processes, but typically there is a need for a further division of the material to a particle size of approximately 1.5 cm, which requires additional supply of energy (Holtzapple et al. 1991). Furthermore, only a modification of lignin takes place, which may be a problem in connection with a possible subsequent enzymatic hydrolysis and fermentation.
Thermal Hydrolysis
Thermal hydrolysis (LHW) is a process (170° C.-230° C.) in which a high dissolution of hemicellulose takes place concurrently with a partial dissolution of lignin and an improved availability of cellulose (for enzymatic hydrolysis). Waste of sugar cane that has not previously been divided and that has been pre-treated with LHW, results in up to 90% of the theoretic ethanol yield after enzymatic hydrolysis and fermentation after addition of moderate amounts of enzyme (Van Walsum et al. 1996). U.S. Pat. No. 4,461,648 describes a method that increases the availability of cellulose- and lignocellulose-containing materials. The method comprises the addition of water steam under pressure, heat treatment and wet explosion. The addition of an oxidizing agent is not described in connection with the described process. A disadvantage of the method is the lack of efficiency when processing higher concentrations of biomass.
LHW has solely been tested with biomass concentrations up to 100 g/l, and it is uncertain, how effective the process is at higher concentrations, which is a necessity for obtaining a economically profitable process—e.g. for the manufacture of ethanol.
Thermal Hydrolysis with Addition of Base and Oxidizer
Finally, U.S. Pat. No. 6,419,788 describes a method of treatment for purification of cellulose from biomass for the manufacture of paper, plastic, ethanol and other chemicals. The process consists of a combination of thermal hydrolysis and wet oxidation, where divided biomass (<1″) is treated under steam pressure (180° C.-240° C.) under addition of an oxidizing agent as well as an alkaline catalyst in a countercurrent reactor. In the treatment a partial dissolution of hemicellulose as well as oxidation of lignin take place, whereby cellulose is purified in the solid fraction. However, this requires washing of the solid fraction under heat and pressure in order to wash-out residue lignin and hemicellulose. The process furthermore includes energy recovery, which contributes to optimize the process economy. However, the combined thermal hydrolysis and wet oxidation do require an energy-costly division of the biomass, and how large a part of the lignin that is actually removed by this process is furthermore unknown.